Exergy maximization of cascade refrigeration cycles and its numerical verification for a transcritical CO 2 eC 3 H 8 system Souvik Bhattacharyya * , S. Bose, J. Sarkar Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, India 721302 Received 28 December 2005; received in revised form 30 October 2006; accepted 1 November 2006 Available online 9 January 2007 Abstract Analysis of an endoreversible two-stage cascade cycle has been implemented and optimum intermediate temperature for maximum exergy and refrigeration effect have been obtained analytically. Further, the heat reservoir temperatures has been optimised independently. A comprehensive numerical model of a transcritical CO 2 eC 3 H 8 cascade system was developed with intent to verify the theoretical results. It is seen that the simulation results agree well for optimal T L but deviate modestly from the theoretical optimum of T H . It has also been observed that system performance improves as T H increases and unlike theoretical predictions, no optimal T H is present within feasible working temperatures. Ó 2006 Elsevier Ltd and IIR. All rights reserved. Keywords: Refrigeration; Simulation; Calculation; Transcritical cycle; Carbon dioxide; R-744; Propane; R-290; Performance Optimisation de l’exergie dans des cycles frigorifiques en cascade et ve ´rification nume ´rique de l’optimisation dans un syste `me au CO 2 eC 3 H 8 Mots cle ´s : Froid ; Simulation ; Calcul ; Cycle transcritique ; Dioxyde de carbone ; R-744 ; Propane ; R-290 ; Performance 1. Introduction In many situations it is desirable to combine two refrig- eration cycles in series in order to obtain a large temperature lift or otherwise to utilize the waste heat to improve system efficiency. These combined refrigeration cycles, also called cascade systems, are used to take operating temperatures far higher and far below the ambient temperature thus opening up possibilities of utilizing the refrigerator simultaneously as a heat pump as well. As a result of this, cascade systems have gained popularity in process industries, which perform both cooling and heating operations simultaneously. Many have used irreversible thermodynamic techniques extensively to optimise refrigeration cycles. Using various heat conduction laws different optimal configurations of the cycles were reported. Yan and Chen [1] investigated multistage refrigeration systems as well as single stage refrigeration cycles and obtained the formulation for * Corresponding author. Tel.: þ91 3222282904; fax: þ91 3222255303. E-mail address: souvik@mech.iitkgp.ernet.in (S. Bhattacharyya). 0140-7007/$35.00 Ó 2006 Elsevier Ltd and IIR. All rights reserved. doi:10.1016/j.ijrefrig.2006.11.008 International Journal of Refrigeration 30 (2007) 624e632 www.elsevier.com/locate/ijrefrig